1. Technical Field
The present disclosure relates to a control device for a resonant apparatus.
2. Description of the Related Art
A resonant apparatus as a resonant converter is known in the state of the art, using a half-bridge or a full-bridge as a switching circuit. In the case of half-bridge resonant converter, the switching circuit comprises a high-side transistor and a low-side transistor connected in series between an input voltage and ground. A square wave having a high value corresponding to the power supply voltage and a low value corresponding to ground may be generated by conveniently switching the two transistors. A small time interval Td called “dead time”, during which the transistors are off, is typically added immediately after each of them is switched off.
In resonant converters, the square wave generated by the half-bridge is applied to the primary winding of a transformer by means of a resonant network which comprises at least one capacitor and one inductor; the secondary winding of the transformer is connected with a rectifier circuit and a filter to provide an output constant voltage. The value of the output voltage depends on the frequency of the square wave.
The so-called LLC resonant converter is often used among the several types of resonant converters, especially the half-bridge LLC resonant converter (the designation comes from the resonant circuit employing two inductors (L) and a capacitor (C)); a schematic circuit of an LLC resonant converter is shown in FIG. 1. The resonant converter 1 in FIG. 1 comprises a half-bridge of transistors Q1 and Q2, with respective body diodes Dbl and Db2, driven by a control device 3 by means of the signals HSGD and LSGD. The common terminal HB between transistors Q1 and Q2 is connected to a resonant circuit 2 comprising a series of a capacitor Cr, an inductance Ls, and a parallel circuit that includes another inductance Lp, connected in parallel to a primary winding of a transformer 10 with a center-tap secondary. The inductance Lp is connected to a sense resistance Rs connected to ground GND and the voltage Vs across the resistance Rs is sensed by the control device 3. The two windings of the center-tap secondary of transformer 10 are connected to the anodes of two diodes D1 and D2, the cathodes of which are both connected to the parallel of a capacitor Cout and a resistance Rout. The output voltage Vout of the resonant converter is the voltage across said parallel, while the output current Tout flows through the resistance Rout. The resonant converter 1 comprises a feedback circuit including an error operational amplifier 6 frequency compensated by means of the circuit element 5 and having an inverting input terminal that receives the voltage Vr=Vout*R2/(R1+R2) and a non-inverting terminal that receives a reference voltage Vref. The output voltage of the operational amplifier 6 is at the input of an optocoupler 4 adapted to provide the voltage Vc to the control device 3.
Resonant converters offer considerable advantages as compared to traditional switching converters (non-resonant, typically PWM-controlled (Pulse Width Modulation)): waveforms without steep edges, low switching losses in the power switches due to “soft” switching thereof, high conversion efficiency (>95% is easily reachable), ability to operate at high frequencies, low EMI (electro-magnetic interference) generation and, finally, high power density (i.e. enabling to build conversion systems capable of handling considerable powers levels in a relatively small space).
However, the same resonant converters are affected by certain disadvantages during the start-up step. In said step, when the high-side transistor Q1 is switched on the first time, the voltage seen by the primary winding is substantially equal to the power supply voltage. In the successive semi-period of the square wave, when the low-side transistor Q2 is switched on, the voltage seen by the primary winding is substantially equal to the voltage across the capacitor Cr; therefore, the current flowing through the resonant network increases more quickly during the on state of the high-side transistor, while decreases less quickly during the on state of the low-side transistor. Thereby, with a 50% duty cycle, when the low-side transistor is switched off again, the current flows through the body diode Db2 thereof When the high-side transistor is switched on again, a reverse voltage is developed across the body diode Db2 of the low-side transistor, while the diode Db2 is still conducting. Under said conditions, the high-side transistor is switched on under hard switching conditions and the diode Db2 is stressed in reverse recovery. Therefore, both the high-side transistor and the low-side transistor are conductive in the same time period by short-circuiting the supply terminal with the ground terminal until the body diode Db2 is recovered. Under such conditions, the voltage at the terminals of the transistor may vary so quickly that the intrinsic, parasitic bipolar transistor of the transistor MOSFET structure may be triggered thus causing a condition of shoot-through which may cause the destruction of the transistor in few microseconds.
A solution to the hard switching problem is known from U.S. Pat. No. 8,212,591 which discloses an apparatus and a method for controlling a resonant switching system that includes a first switch and a second switch in a half-bridge configuration for driving a resonant load. A control system includes a driver for switching on and switching off the switches alternatively according to a working frequency of the switching system. The control system includes a detector for detecting a zeroing of the working current supplied by the switching system to the resonant load in a temporal observation window. The observation window follows each switching off of at least one of the switches, and has a length equal to a fraction of a working period of the switching system. A correction circuit is provided for modifying the working frequency in response to each detection of the zeroing in the observation window.
A further mechanism can be used for the apparatus of U.S. Pat. No. 8,212,591 to prevent hard switching during the start-up step, that is to prevent the hard switching of the high side transistor that occurs during the start-up step at the end of the on time period of the low side transistor.
This mechanism, that consists in synchronizing the oscillator to the zero crossing of the current, modifies the working frequency of the switching system and therefore must be deactivated after the start-up step.